This invention relates to battery chargers and more particularly to battery chargers having means associated therewith for sensing conditions in the battery.
The fast charging of batteries such as, for example, nickel-cadmium cells, involves problems relating to both overvoltage and thermal conditions. Charging beyond a predetermined voltage may cause an unacceptable level of gassing which in turn may damage the separators, and the plates, or the resultant pressure may rupture the cell casing itself. Overheating can also result in increased gassing rates as well as problems from high temperature itself as is well known to the art.
Because of the notoriety of these problems, the prior art is replete with temperature-sensitive charging circuits, voltage sensing charging circuits, and temperature-compensated voltage sensing circuits. In most instances, however, the prior art has not been able to satisfactorily solve the problems which lead to battery damage. Batteries using voltage-sensing means require cells having closely controlled and matched cell voltage characteristics. Even with such ideal battery characteristics, the battery may be charged with these systems at temperatures so high or so low as to cause damage.
The difficulty of solving both voltage and temperature problems by sensing voltage is due, at least in part, to the fact that the operation of the battery charging system may be in widely varying ambient conditions making it difficult, for example, to provide sufficient temperature compensation in a voltage sensing circuit for all possible operation conditions. For example, a temperature compensated voltage sensing battery charging system might still expose the battery to harmful heat levels before the preselected voltage level was reached due to high ambient temperature or as caused by repetitive charge-discharge cycles. Conversely, a battery charging system exposed to a very cold ambient and depending only on temperature as a control means might continue in the overcharge condition without reaching the predetermined temperature cut-off point. A battery charging system in an airplane, for example, may be exposed to a very high ambient temperature on the ground in an equatorial zone and to very low temperatures in flight.